Spectacle lenses, in particular so-called progressive spectacle lenses, are provided with marks, the position of which being detected during the manufacture of the spectacle lens, and being processed, in order to chuck the spectacle lens in a correct position, to work it, to stamp it, and, finally, to bring it into the spectacle frame of the end user. Marks are applied to spectacle lenses in a durable manner, i.e. by diamond scratching methods, or during the forming when plastic material spectacle lenses are molded, or by laser marking. Moreover, the term “mark” as used in the context of the present invention, also comprises other irregularities of the spectacle lens, for example streaks within the glass or the plastic material.
If, in the context of the present application, “spectacle lenses” are referred to, it is to be understood that this term also comprises contact lenses or other comparable optical elements.
In order to avoid that the user of the spectacle is irritated by the marks when the spectacle is used, the marks are configured such that they are visible only under very special light circumstances. The detection of the position of a mark on a spectacle lens during the production process is, therefore, quite difficult. An additional problem is that the spectacle lenses in a production process have very different optical effects due to the specific requirements of the later user of the spectacle. Therefore, within a production line spectacle lenses of highly different optical effects are following one after the other so that these optical effects have to be taken into account in a fast sequence during the subsequent working of individual spectacle lenses.
The recognition of a mark on a spectacle lens is particularly difficult, when the spectacle lens is provided with a phototropic coating. Such spectacle lenses are commonly referred to in the art as HIP (High Index Phototropic) lenses. Phototropic coatings of the type of interest in the present context are relatively thick. The coating thickness is about 30 μm, whereas other lens coatings (antireflex coatings etc.) have a thickness of only about 2-3 μm. The marks are applied to the lens body prior to the coating. Therefore, they are covered by the coating that is applied later. Whereas this does not present a real problem for conventional thin coatings for what concerns the recognition of marks, a problem arises when the mark is covered by a thick phototropic coating. The marks become optically smeared thereby, i.e. the edges of the mark become blurred. In contrast to the situation with thin coatings only a low frequency phenomenon appears.
For a control of progressive lenses both in the far and in the near design reference point, it is necessary to measure the effect of the progressive lenses at predetermined coordinate points on the spectacle lens depending on the applied marks. For a manual or for an automatic measurement these marks must, therefore, be made visible. According to prior art methods and apparatuses, this is done by means of rhombic gratings or strip patterns which are imaged blurred, wherein the edge transitions bright/dark make the mark visible.
A disadvantage of this prior art approach, in particular during the automatic recognition of a mark, is that the grating is imaged differently, depending on the optical effect of the particular spectacle lens under investigation, namely as a function of the particular dioptric effect of the spectacle lens. It is, therefore, necessary to make considerable efforts for the recognition of marks with respect to the algorithms used. Prior art methods, therefore, have not yet matured into a complete, safe and automatic recognition. Under actual practice it is, therefore, necessary that even with automatic control devices a person of particular trained skill has to manually interfere into the production process for correcting wrong recognitions.
But also in a situation where the marks are recognized within a production process by means of a manual examination action, the situation is quite similar. In that case different illuminations are used depending on the particularly used marking process in order to make the marks visible. According to prior art apparatuses this is affected by manually interchanging illumination units. However, also with these manual methods the marks are quite obscure and difficult to recognize so that errors are possible during the positioning and the orienting of the particular spectacle lens. This holds true in particular with regard to the amount of time being available for recognizing the mark. For these reasons, it is necessary, in particular for plastic material spectacle lenses, to additionally mark the spectacle lenses prior to the recognition of the marks as such, namely by means of a felt pen or the like (so-called “pointing”) which results in additional work and time consumption.
Corresponding considerations also apply for other areas within the processing of such spectacle lenses, namely for stamping automats, which, according to the actual state of the art likewise require the assistance of an operating person. The person observes the spectacle lenses on a screen in order to manually correct within the system positions of the marks that have not been properly recognized automatically. This is, for example, affected by means of roller sphere input. This disadvantage likewise results in a reduction in productivity of a video-assisted, manually operated stamping machine.
Document U.S. Pat. No. 3,892,494 discloses a method and an apparatus for locating optical microeffects on optical components, for example lenses. A laser beam is directed on the component under investigation via a beam splitter, namely a semitransparent mirror. The laser beam runs through the component and impinges on the other side thereof on a retroreflector, for example a retroreflecting foil from which it is again reflected back through the component and then runs back on the same ray path until it is deflected by the beam splitter directed to a camera.
A disadvantage of this prior approach is that it may result in problems for spectacle lenses of highly different curvature. Due to the highly different curvature the observation ray path must be long and stopped down in order to achieve a sufficient depth of field. On the other hand, the structures of the retroreflector shall not be imaged sharply because for avoiding wrong interpretations it is desired to have a relatively homogeneous background. As a consequence, the retroreflector in these applications must be located very far behind the plane of the spectacle lens to be measured and, further, it should be configured very large, because strongly negative spectacle lenses would image the retroreflector on a very small scale so that the entire lens could no more be seen over the retroreflector.
In addition, it is important in the context of the present invention that not only the recognition of marks or of other irregularities on spectacle lenses is concerned, but moreover the integration of this recognition process into a measuring instrument or into a working process. In such instances, however, a sensor is arranged behind a spectacle lens, i.e. on the same side as the retroreflector of the prior art apparatus, for measuring physical properties of the spectacle lens. For design reasons it is, therefore, impossible to locate the retroreflector far behind the plane of the spectacle lens.
Document U.S. Pat. No. 4,310,242 discloses an apparatus for measuring the optical qualities of windshields in situ. For that purpose an optical apparatus is also used here having a light source, a beam splitter, a retroreflector positioned behind the windshield to be measured, as well as a camera. A fine pattern is projected through the beam splitter on a retroreflecting screen, such that a real image of this pattern is generated on the retroreflecting screen being deformed by the windshield positioned within the ray path. Via the beam splitter the camera also looks in the direction of projection through the windshield under investigation on the retroreflective screen. Inhomogeneities, tension double refractions, streaks and the like are clearly made visible in such a way.
Document DE 43 43 345 A1 disclosed methods and apparatuses for measuring the reflective and the transmissive, respectively, optical properties of a sample. A measuring radiation is directed on a sample and is reflected by the sample so that it impinges on a retroreflector which, again, sends the measuring radiation back via the object to the light source where a decoupling a detector takes place.
Another similar approach is also described in document EP 0 169 444 A2.
In a prior art vertex refractometer “Focovision SG1” a light beam is emitted from a light source through a green filter and is directed on a spectacle lens to be examined via a beam splitter. The light beam runs through the spectacle lens and impinges on a sensor head located behind the rear side of the spectacle lens. In such a way physical properties of the spectacle lens may be measured. Moreover, on the rear side there is a plane in which exchangeable illumination accessories may be located. These illumination accessories illuminate the spectacle lens from the rear such that the marks become visible. A corresponding observation light beam runs from the spectacle lens to the beam splitter, is there reflected and then fed to a camera via other optical means. In a first illumination accessory a sharply limited bright light bundle is directed on the spectacle lens under a flat angle. Marks that have been applied by scratching will then appear bright due to the irregular shape of the scratch in front of a dark background. A second illumination accessory, in contrast, is provided for spectacle lenses in which the marks have not been applied by scratching but by forming or by laser beams instead. This second illumination accessory therefore, comprises a bright line grating being illuminated from below, as well as a plurality of auxiliary lenses arranged one besides the other by means of which these bright gratings may be imaged to infinity.
This prior art apparatus, therefore, is relatively difficult to operate. Further, the point in which the measuring beam emitted by the light source impinges on the spectacle lens coincides with the point in which the observation light beam exits from the spectacle lens. This may result in errors during the processing.
DE 197 40 391 discloses still another observation apparatus for masked markings, i.e. marks. In this apparatus, a lens, being provided with a masked marking, is illuminated with an illumination light. The masked marking is then observed as a shadow of the lens generated by the observation light.
In this apparatus we have the disadvantage that the marking will be shifted depending on the type of the lens and its local prismatic effect or is reduced in size or amplified, respectively, by the positive or by the negative effect of the lens.